The present invention relates to a semiconductor laser device and a semiconductor laser module using the semiconductor laser device, particularly to a semiconductor laser device free from output reduction due to internal absorption and a semiconductor laser module whose power consumption required for driving is reduced, whose failure rate is halved compared to a conventional failure rate, whose loss can be reduced even when multiplexing a laser beam, and whose maximum optical-fiber output is 180 mW or more, since the semiconductor laser device is built therein.
Because a semiconductor laser device having an active layer (luminous region) comprising a multi-quantum well structure operates at a high optical output compared to the case of a semiconductor laser device having an active layer of a bulk semiconductor, the study for practically using the semiconductor laser device as a light source of a erbium-doped optical-fiber amplifier is progressed. For example, in the case of a laser device to be operated at a range of 1.48 xcexcm, by forming a two-dimensional heterostructure having a lattice-matched multi-quantum well structure using GaInAsP material system on an InP substrate, a buried heterostructure (BH structure) laser device in which a single transverse mode is controlled is practically used as a pumping light source for an optical-fiber amplifier. Moreover, it is requested to further increase the optical output of the semiconductor laser device.
To realize high-output power of the semiconductor laser device having an active layer comprising a multi-quantum well structure, it is effective to increase the cavity length of the laser device. This is because, by increasing the cavity length, a larger current can be injected even with the same threshold current density and thereby, an optical output is raised.
However, when a multi-quantum well structure of an active layer is constituted of a lattice-matching system, a driving current at which optical output power is saturated increases by increasing a cavity length but a problem occurs that reduction of the external quantum efficiency becomes predominant due to internal absorption and an optical output rather lowers. Therefore, in the case of a semiconductor laser device having a multi-quantum well structure of the lattice-matching system, it is avoided to set the cavity length of the semiconductor laser device to 1000 xcexcm or more.
Meanwhile, a pumping light source for an optical-fiber amplifier having the structure schematically shown in FIG. 1 is known. The light source obtains an optical output of 250 mW or more from an optical fiber F by multiplexing optical outputs supplied from two laser modules M and M respectively having an optical output of approximately 140 mW by an optical coupler C. The request for higher output is intensified to the light source.
In the case of the light source shown in FIG. 1, optical outputs supplied from the laser modules M and M are attenuated when multiplexed by the optical coupler C. Therefore, an optical output of a usable light source becomes smaller than an optical output (sum of optical outputs of two laser modules) which can be essentially expected from laser modules used. That is, an energy conversion efficiency which is defined by the ratio of an optical output power from an optical-fiber end to the total driving power of laser modules is lowered. Moreover, the function as a light source disappears if one of the two modules M and M which are used in FIG. 1 fails. That is, the light source is lacking in the reliability as a high-output light source.
Moreover, as a pumping light source in which optical outputs from a plurality of laser modules are coupled, for example, U.S. Pat. No. 5,936,763 discloses an art for fabricating a high-output pumping light source by using a plurality of semiconductor laser modules having oscillation wavelengths different from each other, the oscillation laser beams of which are wavelength-multiplexed by a wavelength-multiplexing coupler.
In this prior art, oscillation laser beams from semiconductor laser devices in semiconductor laser modules are fixed by a diffraction grating and then, wavelength-multiplexed by a wavelength-multiplexing coupler.
However, when fixing (or locking) an oscillation wavelength of a semiconductor laser device, it is difficult to stably control an optical output because the change in driving current and in ambient temperature could cause the longitudinal modes of the oscillating laser light of the semiconductor laser to shift accordingly thereby giving rise to kinks in light output versus injection current characteristics as shown in FIG. 2.
As a laser module for solving such problems, the following semiconductor laser module is disclosed in U.S. Pat. No. 5,845,030.
That is, the module disclosed in the U.S. patent has a structure obtained by arranging optical fibers with a grating formed therein at the output facet of a semiconductor laser device. In this case, a reflection bandwidth of the grating is set to a value larger than a wavelength interval of longitudinal modes of a laser beam oscillated from the semiconductor laser device, specifically to a value of 2 nm or more so as to mitigate the influence of the longitudinal-mode fluctuation on an optical output from an optical fiber of the module when the semiconductor laser device is driven.
The above-described grating shows a reflection spectrum having a reflectance only in a wavelength range about a specific wavelength (xcex0) as shown in FIG. 3. In FIG. 3, the wavelength width between the wavelength at which the reflectance is half the peak reflectance is referred to as reflection bandwidth.
However, when wavelength-multiplexing is performed by the module of the prior art, the following problems occur.
That is, as a result of setting the reflection bandwidth of the grating to 2 nm or more, the spectral width of a laser beam output from the optical fiber of the module increases and thereby, the number of oscillated wavelengths which can be multiplexed by a wavelength-multiplexing coupler having a narrow transmission band decreases, a degree of multiplexing lowers, and the loss of the wavelength-multiplexing coupler increases.
It is an object of the present invention to solve the above-described problems of a semiconductor laser device having a multi-quantum well structure and to provide a semiconductor laser device in which an optical output is not lowered even if a cavity length is set to a value larger than 1000 xcexcm.
Moreover, it is another object of the present invention to provide by using the above-described semiconductor laser device a semiconductor laser module in which the device driving currents can be decreased to provide high driving reliability, and wavelength-multiplexing coupler has a small loss in wavelength-multiplexing. The semiconductor laser module of the present invention can be used as a high-output pumping light source for an optical-fiber amplifier compared to a conventional pumping light source which has a maximum optical-fiber output of 180 mW or more, for example.
To achieve the above-described objects, the present inventors made intensive studies and in the course of their research, they noticed the fact that an optical output of a semiconductor laser device is specified by an integrated value of an efficiency and an injected current density of the semiconductor laser device and examined the relation between efficiencies and cavity lengths of a semiconductor laser device having a lattice-matching-system quantum well structure and a semiconductor laser device having a lattice-mismatching-system quantum well structure with respect to the noticed fact. As a result, they had an idea that it is possible to fabricate a semiconductor laser device for emitting light at a high output compared to a conventional device by setting a cavity length to a value equal to or more than a certain value and then, further continued the studies and finally developed a semiconductor laser device and a semiconductor laser module using the semiconductor laser device of the present invention.
That is, in the semiconductor laser device of the present invention, a stacked structure of a semiconductor including an active layer comprising a strained multi-quantum well structure is formed on a substrate, a cavity length is larger than 1000 xcexcm but 1800 xcexcm or less, a low-reflection film having a reflectance of 3% or less is formed on one facet, and a high-reflection film having a reflectance of 90% or more is formed on the other facet. Particularly, a semiconductor laser device which has a compressed strained multi-quantum well structure in which the strained multi-quantum well structure has a lattice-mismatching rate of 0.5 to 1.5%, preferably 0.5 to 1.1% is provided.
Moreover, the present invention provides a semiconductor laser module in which a semiconductor laser device is sealed in a package while set to a cooling device comprising Peltier elements (thermoelectric effect elements) and an optical fiber is opposed to the output facet of the semiconductor laser device. The present invention provides a semiconductor laser module in which the cooling device is preferably constituted by electrically alternately arranging 40 pairs or more of p-type and n-type conductive Peltier elements and holding these elements by top and bottom ceramic substrates and in which a grating having a reflection bandwidth of 2 nm or less, preferably 1.5 nm or less and larger than the longitudinal-mode wavelength interval of a laser beam oscillated from the semiconductor laser device is formed on the fiber. In the subsequent description, the number of pairs of a p-type and an n-type Peltier elements is defined as xe2x80x9cthe number of pairsxe2x80x9d.